The overall objective of this project is to determine the physiological relevance of the NKG2D receptor in NK cell-mediated and T cell-mediated immune responses. In prior studies, we have implicated this receptor in NK cell-mediated anti-viral and anti-tumor immunity, and have uncovered an important role for NKG2D in the pathology associated with type I autoimmune diabetes. In this competitive renewal application, we will focus our efforts towards defining the mechanisms of NKG2D-dependent activation of T cells and NK cells in model systems of immunity to tumors and pathogens and in autoimmunity. Specific aims are: 1. To determine the mechanisms whereby NKG2D and its ligands contribute to autoimmune diabetes in the NOD mouse model; 2. To develop genetic models for selective deficiency of NKG2D in discrete cell lineages and to determine the effects on innate and adaptive immune responses; and, 3. To test the hypothesis that NKG2D costimulation in human and mouse T cells is restricted to unique T cell subsets or activation states. Aim 1 wi|l evaluate the potential role for NKG2D in models of autoimmunity that are regulated by NK cells, CD8+ cells (e.g. the 8.3 TcR-transgenic model), and CD4+ T cells (e.g. the BDC2.5 TcR-transgenic model). In addition, we will test whether anti-NKG2D mAb treatment can affect autoimmune manifestations other than diabetes. In aim 2 we will generate a conditionally NKG2D-deficient mouse on the C57BL/6 background in order to test the hypothesis that NKG2D is important in immunity against autoantigens, tumors and pathogens. An important goal is to determine in model systems where the function of NKG2D is critical --whether NKG2D is required in NK cells, T cells, or both cell types. Specific aim 3 will determine why NKG2D is unable to costimulate freshly isolated human NKG2D+ T cells or short-term activated mouse NKG2D+ T cells, but is able to efficiently costimulate long-term cultured human and mouse CD8+ T cells and clones, as well as CD8+ T cells isolated from tissue undergoing an autoimmune reaction in vivo.